1. Field of the invention
The present invention relates to a liquid crystal display device using a thin film transistor (hereinafter referred to as a TFT) as a switching element, and more particularly to a light shielding method of a TFT located in a pixel.
2. Description of the Related Art
In general, a liquid crystal display device includes a TFT array substrate in which TFTs are mounted, a counter substrate which is located opposite to the TFT array substrate, a liquid crystal layer interposed between the TFT array substrate and the counter substrate, a polarizing plate provided on the side of the TFT array substrate opposite to the liquid crystal layer, and a polarizing plate provided on the side of the counter substrate opposite to the liquid crystal layer.
According to the liquid crystal display device, when illumination light incident from the counter substrate side transmits through the liquid crystal layer, transmittance of the illumination light is changed along with a change in orientation of liquid crystal molecules in accordance with a signal from a pixel electrode so that light having an intensity necessary to display an objective image is emitted.
When the illumination light incident into the liquid crystal display device enters into the channel formation region of a TFT, the conductivity is increased by a photoelectric effect so that it becomes a leak current (this is called a light leak current), thereby causing malfunction of the TFT In general, in order to prevent from light entering into the channel formation region, in an active matrix liquid crystal display device, a light shielding layer made of a metal material such as Cr (chromium) or Al (aluminum), a resin material, or the like is formed on the side of the counter substrate opposite to the TFT array substrate.
Most of light which transmits through the liquid crystal layer is prevented from entering into the channel formation region by the light shielding layer provided on the counter substrate side or the TFT array substrate side. However, a portion of transmitting light becomes reflection light (return light), which is reflected on an interface and the like of the TFT array substrate and enters into the TFT array substrate again so that it is incident into the channel formation region to cause a light leak current. When the amount of charges in a storage capacitor or the like is changed according to the light leak current so that an applying voltage to a pixel electrode is changed, light transmittance in liquid crystal is changed therewith. Thus, there is a problem in that a contrast ratio is reduced or color heterogeneity of a display image and improper display are caused. In particular, in halftone display, a region in which a change in light transmittance is significantly affected by a change in applying voltage is used. Thus, failure of display resulting from the light leak current in the TFT is more visible.